Enzymatic Strategy for the Resolution of New 1‐Hydroxymethyl Tetrahydro‐β‐carboline Derivatives in Batch and Continuous‐Flow Systems

摘要

Many alkaloids containing a tetrahydro‐β‐carboline skeleton have well‐known therapeutic effects, leading to increased interest in the synthesis of these natural products. Enantiomers of N‐Boc‐protected 1‐hydroxymethyl‐1,2,3,4‐tetrahydro‐β‐carboline [(±)‐>7], 1‐hydroxymethyl‐6‐methoxy‐1,2,3,4‐tetrahydro‐β‐carboline [(±)‐>8], and 1‐hydroxymethyl‐6‐fluoro‐1,2,3,4‐tetrahydro‐β‐carboline [(±)‐>9] were prepared through enzymecatalyzed asymmetric acylation of their primary hydroxyl group. The preliminary experiments were performed in a continuous‐flow system, while the preparative‐scale resolutions were done as batch reactions. Excellent enantioselectivities (E>200) were obtained with Candida antarctica lipase B (CAL‐B) and acetic anhydride in toluene at 60 °C. The recovered alcohols and the produced esters were obtained with high enantiomeric excess values (ee≥96 %). The O‐acylated enantiomers [(S)‐>10–(S)‐>12)] were transformed into the corresponding amino alcohols [(S)‐>7–(S)‐>9)] with methanolysis. Microwave‐assisted Boc removals were also performed and resulted in the corresponding compounds (R)‐>4–(R)‐>6 and (S)‐>4–(S)‐>6 without a drop in the enantiomeric excess values (ee≥96 %).